Many audio systems today can be used with stereo headphones or earphones (collectively “headphones” hereinafter). In fact, many existing and emerging classes of audio systems today are intended to be used either predominantly or only with headphones, and this is particularly so for systems that are designed to be portable or wearable. Some noteworthy examples today are MP3/4 music players, CD players, and personal sound recorders/players generally. In these systems especially, and as part of an ongoing trend towards smaller and more efficient electronic devices, reducing overall system size and power consumption are often also important considerations.
In most of these audio systems a 3-wire signal feed to the headphones is employed, wherein each of two stereo signals is carried via an individual wire and a third common wire. The connection points for these wires to the audio system proper are usually termed a “left signal terminal,” a “right signal terminal,” and a “common load terminal.” The left and right stereo signals thus pass through the left and right signal terminals, and a voltage used to bias the speaker elements within the headphones is present at the shared common load terminal. Usually the bias voltage is designed to be nominally half of the main direct current (DC) supply voltage, since this permits achieving maximum dynamic range during sound playback.
Unfortunately, designing a mechanism to suitably bias the common load terminal in this manner in an audio system can be problematical, especially for a 3-wire signal feed to headphones. This is because it is usually necessary to concurrently prevent DC from flowing through the common load terminal to the system ground. Since the common load terminal is in the current path through the headphones and the signal amplifiers that drive them, any undue current flow here can potentially damage these elements and is generally wasteful of power.
FIGS. 1-3 (prior art) are schematic block diagrams depicting some typical headphone driver circuits. One common prior art circuit 10 for this application is shown in FIG. 1. Here, left and right sub-circuits (shown stylistically simply as signal amplifiers 12, 14) provide left and right channel stereo signal content, via left and right signal terminals 16, 18, to left and right speaker elements 20, 22. The speaker elements 20, 22 are both further connected to a common load terminal 24, thus forming the typical 3-wire configuration discussed above.
A key element in circuit 10 is a capacitor 26, which connects the common load terminal 24 to a system ground 28. This capacitor 26 performs DC blocking, preventing the undesirable flow of DC from the common load terminal 24 to the system ground 28. Concurrently, however, the capacitor 26 must still generally permit desired signal content (as alternating current (AC)) to flow through the speaker elements 20, 22 and to the system ground 28. Theoretical current paths 30 are stylistically depicted in FIG. 1 in ghost outline.
To pass the full desired frequency range through the speaker elements 20, 22 the capacitor 26 usually needs to have quite a high value, and this is a major problem with circuit 10. For example, in common audio design practice it is desirable to pass 25 Hz stereo signal content with less than 3 dB of attenuation and to pass 100 Hz content with essentially no attenuation. For the sake of this example some common supply voltage levels and headphone impedances are also shown in FIG. 1. From all of this it follows that the value of the capacitor 26 usually needs to be on the order of 200-400 μF (micro farads). The capacitor 26 thus tends to be physically large and expensive, and tends to appreciably increase the size and cost of audio systems that employ this approach.
FIG. 2 shows the most common prior art circuit 40 for driving headphones, albeit one conceptually much the same as the approach of circuit 10. Rather than use one DC-blocking capacitor, two capacitors 42a, 42b are used here instead, one per audio channel. Unfortunately, for readily apparent reasons, circuit 40 suffers from the same problems as circuit 10.
FIG. 3 shows yet another prior art circuit 50 that is frequently used for common load terminal biasing. Here the need for a DC-blocking capacitor in the path connecting the headphones to ground is eliminated by the use of a third amplifier, a bias amplifier 52, which actively drives the bias level of the common load terminal 24. The amplifiers 12, 14, 52 in this arrangement drive the loads presented by the speaker elements 20, 22. Amplifier 52 holds the DC bias on the common load terminal 24.
The approach in circuit 50 conveniently eliminates the need for one or more large, expensive DC-blocking capacitors that consume circuit board space and cause low-frequency performance degradation. Since circuit 50 biases the differential outputs of the amplifiers 12, 14, 52 at mid-supply, there advantageously is no resulting net DC voltage across the speaker elements 20, 22. But the use of three amplifiers 12, 14, 52 in this manner to drive the loads presented by the speaker elements 20, 22 also roughly doubles the power required and which then has to be dissipated. This is generally undesirable and for some applications, such as battery powered audio systems, is a severe disadvantage.
Accordingly, what is needed is an approach to load biasing that does not require substantial additional power yet still eliminates the need for DC-blocking capacitors, i.e., a more efficient approach to load biasing.